| Titel | The Trusted Domain Project OpenDMARC 1.4.2 Improper Authentication |
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| Beschreibung | OpenDMARC 1.4.2 incorrectly expands the SPF `%{l}` macro to the fixed string `postmaster` when using its built-in SPF checker, `SPFSelfValidate`. For a non-null envelope sender, `%{l}` should expand to the local part of the checked identity. OpenDMARC instead applies the special null-sender substitution to every message.
As a result, an SPF mechanism such as `exists:%{l}._spf.%{d}` performs the DNS lookup for `postmaster._spf.example.com` rather than the actual sender-specific name, such as `alice._spf.example.com`. The domain portion is expanded correctly, but the authorization decision is made using the wrong local identity.
A remote unauthenticated attacker can exploit this behavior against a domain that uses `%{l}` for per-user SPF authorization. If the fixed `postmaster` lookup succeeds while the actual sender-specific lookup would fail, OpenDMARC may return `spf=pass`. When the SPF domain aligns with the visible `From:` domain, the incorrect result may also produce `dmarc=pass`, allowing forged mail to bypass a published `p=reject` policy.
The issue can also cause legitimate messages to fail because OpenDMARC never consults the sender’s actual per-user SPF entry. OpenDMARC should derive `%{l}` and `%{s}` from the checked identity and use `postmaster` only when the reverse path is null. |
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| Quelle | ⚠️ https://xuyongzhe-vt.github.io/share/opendmarc-spf-postmaster-macro.html |
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| Benutzer | yongzhe_xu (UID 100420) |
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| Einreichung | 07.08.2026 06:15 (vor 2 Monaten) |
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| Moderieren | 28.09.2026 13:53 (2 months later) |
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| Status | Akzeptiert |
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| VulDB Eintrag | 411038 [Trusted Domain Project OpenDMARC bis 1.4.2 SPF Macro opendmarc_spf.c opendmarc_sp2_find_mailfrom_domain schwache Authentisierung] |
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| Punkte | 20 |
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